Millimeter to Inch Formula
in = mm ÷ 25.4
Conversion factors follow SI relationships published by BIPM and NIST, where those standards apply.
Equivalent: in = mm × 0.03937007874015748. 25.4 mm/in is exact (international inch). Reverse: mm = in × 25.4.
Millimeter to Inch Converter
Quick Answer
1 mm = 0.0393701 in
Formula: Inch = Millimeter x 0.0393701
Quick values
Sources
- NIST SI units of length — Meter as the SI base unit of length; derived units such as the millimeter.
- NIST Handbook 44 Appendix C (international inch) — 1 in = 25.4 mm exactly, from the 1959 international yard and pound agreement.
- BIPM SI base units — Official SI base-unit definitions used to derive conversion factors.
Worked examples
- 6 mm hex key: 6 ÷ 25.4 = 0.23622 in (≈ 15/64 in)
- 25.4 mm reference: 25.4 ÷ 25.4 = 1 in exactly
- 100 mm stock: 100 ÷ 25.4 = 3.93701 in
What is millimeters to inches?
The international inch is exactly 25.4 millimeters (1959 yard and pound agreement). Convert millimeters to inches by dividing by 25.4.
Formulain = mm ÷ 25.4
Key conversions
- 1 in = 25.4 mm exactly
- 1 mm = 0.03937007874 in
- 10 mm = 0.3937007874 in
How many inches is 1 millimeter?
One millimeter is exactly 1/25.4 inches, or 0.03937007874015748 in. The 1959 international yard and pound agreement fixed the inch at 25.4 mm; NIST and the BIPM both treat that factor as exact. Machining drawings, fastener standards, and PCB footprints still switch between the two on a single sheet.
Prefer dividing by 25.4 over multiplying by 0.03937 when you need more than four decimals. 10 mm ÷ 25.4 = 0.3937007874 in, which is 25/64 in to the nearest common fractional drill. This calculator uses the exact reciprocal so 25.4 mm returns 1 in with no rounding drift.
Category hub: Length Conversion · Conversion charts · Conversion formulas
Millimeter to Inch Conversion Table
Shop and drawing sizes that switch between mm and inches
| Millimeter (mm) | Inch (in) |
|---|---|
| 1 mm | 0.0393701 in |
| 3 mm | 0.11811 in |
| 5 mm | 0.19685 in |
| 6 mm | 0.23622 in |
| 8 mm | 0.314961 in |
| 10 mm | 0.393701 in |
| 12 mm | 0.472441 in |
| 16 mm | 0.629921 in |
| 20 mm | 0.787402 in |
| 25.4 mm | 1 in |
| 50 mm | 1.9685 in |
| 75 mm | 2.95276 in |
| 100 mm | 3.93701 in |
25.4 mm is exactly 1 in. Fractional-inch notes in the examples are rounded to the nearest 1/64 in.
Browse all conversion charts → · Browse conversion formulas →
Practical use cases for Millimeter to Inch
Furniture and Home Decor
Measuring furniture dimensions and room layouts when working with international suppliers or following tutorials from different countries. European furniture catalogs use centimeters, while US catalogs use inches.
Construction and Carpentry
Working with building plans, materials, or tools from different measurement systems. Converting between metric and imperial units is essential when using international building codes or importing construction materials.
Crafting and DIY Projects
Creating patterns, measuring materials, and working with international tutorials. Many crafting patterns and DIY guides come from different countries, requiring conversions between measurement systems.
Electronics and Displays
Screen sizes for televisions, monitors, and mobile devices are typically measured in inches worldwide, while device dimensions may be specified in centimeters or millimeters in technical specifications.
Fashion and Textiles
Converting between sizing systems for international clothing, fabric measurements, and pattern making. European and Asian sizing often uses centimeters, while US sizing uses inches.
Sports and Fitness
Measuring athletic performance, equipment dimensions, and training distances. Track and field events use meters, while many US sports use feet and yards, requiring conversions for international competitions.
Millimeter to Inch FAQs
Common questions about converting millimeter to inch.
How many millimeters are in an inch?
Exactly 25.4 mm. That is a defined value, not a measurement. One millimeter is therefore 1/25.4 in ≈ 0.03937007874 in. Older US survey inches (before the international inch) differ at the parts-per-million level and are not used on this page.
Why do some charts say 1 mm = 0.0394 in?
That is 1 mm rounded to four decimals. The error is 0.076%. Fine for a tape-measure note, not for a press-fit or a 0.1 mm PCB trace. Use mm ÷ 25.4 and round only the final display.
How do I convert millimeters to a fractional inch?
Convert to a decimal inch first (mm ÷ 25.4), then multiply by 16, 32, or 64 and round to the nearest count. Example: 8 mm ÷ 25.4 = 0.31496 in × 32 = 10.08 → 10/32 in = 5/16 in. State the denominator you rounded to.
Is a metric M8 bolt the same as 5/16 inch?
No. M8 is 8 mm major diameter (0.3150 in). 5/16 in is 0.3125 in (7.9375 mm). They are close enough to confuse a hardware bin and far enough to damage a thread. Convert the diameter, then match the thread standard (ISO metric vs Unified).
Does 1 cm to inches use the same factor?
Yes. 1 cm = 10 mm = 10/25.4 in ≈ 0.39370 in. The centimeters-to-inches converter applies the same international-inch definition; this page stays in millimeters for machining and drawing callouts.
Charts and printable PDF
Use the matching millimeter / inch chart when you need a printable table or offline reference.
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